1. Field of the Invention
The present invention relates to an intake/exhaust valve control system for a combustion engine and, more particularly, to an intake/exhaust valve control system which can variably control the valve timing and valve lift over the whole engine running range.
2. Description of Related Art
Generally, the opening and closing of an intake/exhaust valve are controlled by a camshaft which is driven by a crankshaft connected to the camshaft by a chain or timing belt. Particularly, the design of the camshaft results in valves being opened and closed at a controlled rate of speed, as well as at a precise time in relation to the piston position.
Particularly, the design of the cam lobe contour has a major effect on engine performance. The amount of valve opening, how long the valve remains open (duration), when the valves open and close (valve timing), and the speed at which valves open and close are all determined by the cam lobe design.
The valve timing, valve duration and valve speed are fixed in accordance with the design of the cam lobe.
Therefore, it is impossible to vary the above three factors in response to the variation of driving condition of the vehicle.
That is, with the above fixed three factors, although a maximum output with a low fuel consumption ratio can be obtained at specific engine RPM ranges, but not at other engine RPM ranges.
Therefore, there is a need for a valve control system of an internal combustion engine that can variably control the valve timing and the valve opening duration (valve lift) in response to variation of a driving condition of the vehicle including engine RPM variation.